T-Mobile’s Un-carrier 8.0 strategy, announced today, will give customers the ability to bank unused monthly mobile data allowances for future use. To kick off the offering, dubbed Data Stash, the company is giving all customers 10 gigabytes at no charge.

On a webcast announcing Un-carrier 8.0 today T-Mobile CEO John Legere quoted the slogan the company will use for the new offering. “It’s your data,” he said. “What you don’t use you won’t lose.”

Data Stash
About 120 million U.S. smartphone users are “data guessers” who don’t know how much data they use, Legere said. Of those, 13 million pay overage charges to their carriers, who collectively earn $1.5 billion annually in penalties.

At the other end of the continuum are people who fear overages and “buy up,” subscribing to a higher service tier than needed in order to avoid overages, Legere explained. Until now T-Mobile has not charged for overages but has slowed data rates when customers exceed their monthly data allotment.

Other information from today’s webcast:

T-Mobile’s market share now stands at 17%, up from a previous 11%

“Wideband” LTE is the term T-Mobile uses for LTE service delivered over two 15 MHz spectrum blocks, now available from T-Mobile in some markets. According to Legere the service doubles the bandwidth available to end users